Added some sharing-specific filesystem attributes.
Added todo file
Incoming shares now readable via filesystem

The client now understands shares coming from other accounts, as far as
reading them goes. It does not know anything about access rights or
share ownership. It can understand and decrypt share keys.

No tests for this yet, since there is no code that actually creates
shares. Tests will come once both halves of the concept are in place.
Todo: support contentless and 0-byte files
Empty files are now supported

You just cannot download them.
Empty file downloading now works

Even though Mega does not give us empty file contents, we can just
synthesize them (hint: the file is empty).
Improved test data creation: consider folders

The test data creation functionality now also ensures that the files are
in the expected locations.